As is well known, in order to maintain the structural integrity of the turbine blades of a gas turbine engine, it is necessary to keep the metal temperature below the extreme temperatures of the gas path. Various cooling techniques, such as impingement cooling, trip strips, pedestals, and the like are used to cool the airfoil section by flowing coolant air internally of the blade. The cooling air, which is at a higher temperature than it was initially, is still sufficiently cool to afford cooling of the external surface of the blade. To this end, the air is discharged through film cooling holes in the airfoil's exterior walls and is judiciously controlled to effectuate film cooling of the airfoil's exterior surface.
Ideally, the entire surface of the airfoil should be covered with a sheet of film cooling air while utilizing the least amount of cooling air as possible. Obviously, inefficient use of cooling air adversely affects engine operating performance. As is well known, this cooling air would otherwise be air that is part of the engine working fluid since energy is already put into the air by virtue of its being pressurized by the compressors of the engine. Since this air is extracted from the compressor and hence, is a loss to the gas path or engine's working medium, it bears a large influence in engine efficiency.
It is the goal of the turbine designer within the constraints of the limited amount of air allocated for turbine blade cooling to completely bathe the airfoil surface with a film of cooling air. This has met a modicum of success but still hasn't attained the ideal. Obviously, as engine requirements become more demanding the optimum use of cooling air presents a more challenging problem to the turbine blade designer.
Some examples showing attempts for attaining effective film cooling are disclosed in U.S. Pat. No. 4,676,719 granted to T. A. Auxier, a co-inventor of this patent application, on Jun. 30, 1987 and entitled "Film Coolant Passages for Cast Hollow Airfoils", and U.S. Pat. No. 4,738,588 granted to R. E. Field, on Apr. 19, 1988 and entitled "Film Cooling Passages with Step Diffuser", both patents being assigned to United Technologies Corporation, the assignee common with this patent application.
In one of these examples, the patent discloses a longitudinal slot that intersects the film coolant passage to meter the coolant flow. This serves to allow the designer to select the requisite area of the intersecting holes to regulate and control the amount of coolant being used for film cooling the entire blade. Obviously, since the turbine blade designer is allocated a given amount of air to be used for cooling purposes, by proper control of the metering means, the amount of cooling air in the film cooling passages is regulated and hence, is used effectively.
In the other example, the patent teaches the use of a step diffuser to attain wider diffusion angles of the discharging film cooling air in an attempt to spread the coolant as it leaves the film cooling passage as wide as possible. This serves to cover a wider area of the airfoil surface with a given number of film coolant air holes.
Obviously, one of the criteria for effective film cooling is to prevent the fluid discharging from the film coolant passages to penetrate beyond the boundary layer of the gases adjacent the exterior surface of the airfoil. Hence, in addition to all the other criteria the turbine blade designer must adhere to, he must take the necessary steps to assure that the cooling air penetration into the gas path or mainstream is minimized. For example, if the injected air penetrates beyond the boundary layer, the decay rate of the film will be accelerated due to increased mixing of the film cooling air and the hot mainstream gases. As a consequence the film cooling effectiveness is adversely affected.
Shaped holes, rather than round holes, as disclosed, for example, in U.S. Pat. Nos. 4,676,719 and 4,738,588, supra, and the diffused passage as disclosed in the above mentioned 4,738,588 patent are attempts to attain high levels of film cooling effectiveness. Notwithstanding the modicum of success these techniques have met, they still have shortcomings and are not the optimum for attaining film cooling effectiveness.
We have found that we can improve film cooling effectiveness for certain film cooling holes that are utilized in certain sections of the airfoil. For example, and not by way of limitations, the suction aided film cooling, as taught by this invention, is particularly efficacious when used on the suction side and the trailing edge of the airfoil.